Strain-measuring device

ABSTRACT

The invention relates to a miniature, integrated strain-measuring device with a simple and economical design which can be mass produced at least cost and comprising reliable, reproducible and tamperproof settings. The inventive device can be quickly installed on the structure to be measured without the need for a specialist technician and without altering said structure. The device comprises a deformable element ( 20 ) which is mounted between two mounting posts ( 30, 40 ) which are intended to be fixed to a structure ( 1 ) to be measured, said deformable element ( 20 ) bearing strain gauges ( 61 ) which are connected to an electronic signal-conditioning circuit ( 70 ). The invention is characterised in that it comprises means of prestressing ( 80 ) the deformable element ( 20 ), said means being provided with at least one prestressing rod ( 81 ) which extends between the two mounting posts ( 30, 40 ). The aforementioned rod is designed to move the two mounting posts ( 30, 40 ) closer together in translation and to impose a prestressing curve on said deformable element ( 20 ). The above-mentioned prestressing means ( 80 ) also comprise a compensation member ( 85 ) which allows the two mounting posts ( 30, 40 ) to move in translation in relation to one another when the deformable element ( 20 ) is prestressed, such that measurements can be taken on the structure ( 1 ). The invention is suitable for any application requiring the measurement, inspection, detection or monitoring of deformation in structures, engineering works and mechanical parts, etc. which are subjected to stresses.

The present invention concerns a strain-measuring device including atleast a deformable element placed between at least two mounting contactsdefining a fixation surface for being fixed on a structure to bemeasured, means for prestressing the aforementioned deformable element,means for measuring the stress undergone by the aforementioneddeformable element, and means for treating signals.

Strain measurements consist of detecting deformations of a structureunder the effect of stresses due to applied forces, to the relaxation ofresidual stresses and/or to thermal dilations. The measurement of theelongation or of the contraction of a surface of this structureaccording to a known approach is normally realized by means of adeformable element prestressed or not, for example an elastic strip,fixed at its end in two points of fixation of the surface to bemeasured. The displacement of the ends to the deformable element,representative of the displacement of the two points of fixation of thesurface to be measured, modifies the radius of curvature of thedeformable element. For realizing a device for measuring strain, onecouples, to this deformable element, means for measuring capable ofmeasuring the strains that it undergoes. These means for measuring areconstituted by for example a vibration sensor measuring the particularfrequency of vibration of the deformable element coupled to the means ofexciting into vibration, a contactless sensor measuring the height ofthis deformable element, one or several strain gauges glued on one orthe two faces of this deformable element, etc.

In U.S. Pat. No. 5,353,645, the device for measuring the deformation islodged in a boring of a structure to be controlled and includes adeformable element extended across this boring and coupled at one of itsends to a pressure or force sensor and at the other end to theaforementioned structure. This sensor can be arranged in a screwedcontact on the structure in a manner to place the deformable elementunder stress. Nevertheless, this mode of prestressing cannot be realizedexcept on the structure to be controlled, and is neither reproduciblenor controllable.

The U.S. Pat. No. 4,314,481 describes a device for measuring tensilestress using a flange fixed on a piece to be controlled by means of afixation screw and coupled to a piezoelectric sensor arranged to theright of one of the points of support of the flange on the piece. Thispiezoelectric sensor is constituted by two superimposed piezoelectricwashers and sensitive to shearing. To be able to function, this deviceis placed under force by the fixation screw. In this case, the prestressis directly tied to the torque of the fixation screw and does notproduce the same effects that this applies to a deformable element.

In the publication FR-A-2,701,317, the bended deformable element is afolded strip in Omega shape. This particular form requires folding zonesdifficult to reproduce. Because of this fact, the mechanicalcharacteristics of this strip are different from one another. The directfixation of the ends of the strip on the surface to be measured can bedifficult and permits no conservation of preadjustments. No control ofthe prestress is possible with this particular form of strip.Furthermore, this design does not permit realization of astrain-measuring device integrated in a single piece but necessitates anassembly more or less crafted in several pieces, hence a complex andcostly implementation.

The publication WO-A-00/57149 proposes a strain-measuring device in theform of an integrated combination but still presenting certaindisadvantages. The mode of fixation of the deformable element in themounting contacts causes a relatively large moment that requires fixingthese mounting contacts on the surface to be measured by means of screwsof relatively large diameter thus effecting non negligible tappings inthe structure under stress. This moment prevents the fixation ofmounting contacts by simple gluing. This is because this device isdesigned to interpose wedges under the mounting contacts, receiving thefixation screws of the mounting contacts, these wedges capable of beingglued on the surface to be measured. Nevertheless, they considerablyaugment the gluing surface. By this type of mounting, the deformableelement is separated from the surface to be measured, entailing asensibility to parasitic deformations and a large calibration error inflexing. The mode of fixation of the deformable element is a hyperstaticmounting, but with a mechanical sensitivity somewhat weak and a highsensitivity to temperature, making its compensation very difficult. Thisdevice includes a means for adjusting the prestress. However, theadjusting of the prestress, which defines the characteristics of theextensometer, is not possible except after its fixation on the surfaceto be measured or on any support and is thus not reproducible.Furthermore, no device permits conservation of this adjustment afterdismantling and/or during transport. Finally, the design of the devicepermits integration of only one basic conditioning electronic circuit,and its assembly remains somewhat complex.

The strain-measuring device described in publication WO-A-99/41565essentially concerns a device mass produced, at lower cost and intendedto be glued on a surface to be measured by allowing very large dilationsof this surface, notably thermal. The prestressed deformable element isblocked during transport by means that do not ensure the adjustment ofthe prestress. The adjustment of the prestress cannot be effected exceptby a supplemental device exterior to the device. However, thisadjustment is not sufficiently reliable and necessitates a recalibrationin situ. The balance of forces is only realized very approximatelybecause the compensation spring working in tensile stress is notregulatable. One can correct the forces only in a direction, requiringpositioning of the deformable element on simple supports. Theconsequences of this are a difficult adjustment and a positioning of thedevice sensitive to the shocks and to the variations of the functioningcycle. Linearity and hysteresis are strongly influenced by the qualityof the supports and thus very difficult to control.

Strain measurements can be used in a multitude of applications such asin equipment for security, regulation and/or control for

-   -   measuring forces applied to a structure,    -   surveilling the deformations of a work or of a structure under        service or accidental loads,    -   measuring the drive or breakage coupling in a vehicle,    -   weighing,    -   controlling overloading.

Nevertheless, considering their disadvantages, the techniques currentlyused in the strain-measuring devices do not permit generalization of theuse in these applications.

The present invention aims to compensate for these disadvantages andproposes a miniature, integrated strain-measuring device, of simpledesign and economical, capable of being mass produced at lower cost, ofwhich the adjustments are reliable, reproducible and not disajustable,capable of being rapidly installed on a structure to be measured withouta specialized technician and without modifying the structure.

Toward this end, the invention concerns a strain-measuring device of thekind indicated in preamble characterized in that the prestressing meansextend between at least the two mounting contacts and are arranged fordrawing together these two mounting contacts in translation and imposinga curvature of precise prestress to the aforementioned deformableelement, the prestressing means being also arranged for allowing arelative displacement in translation of these two mounting contacts whenthe aforementioned deformable element is prestressed.

In one form of preferred implementation, the prestressing means arefreely mounted in translation in one of the mounting contacts and aretied at least in translation to the other mounting contact. Theseprestressing means can include at least a prestressing rod, of which atleast the part tied in translation to the other mounting contact isthreaded and cooperates with at least a nut for displacing theaforementioned mounting contact and at least a compensation memberarranged for exercising a return force between the aforementionedprestressing rod and one of the mounting contacts.

In a variant implementation, the prestressing means can be tied at leastin translation to the two mounting contracts. These prestressing meanscan then include at least a prestressing rod, of which at least theparts tied in translation to the aforementioned mounting contacts arethreaded in an opposite direction, at least two nuts arranged forreceiving the aforementioned threaded parts from the prestressing rodand at least a compensating member arranged for exercising a returnforce between the aforementioned mounting contacts.

According to the chosen implementations, the nut can be integrated withor coupled to the corresponding mounting contact. It can also beconstituted from a tapped boring provided in the aforementioned mountingcontact. One can also provide a supplementary nut coupled on to the endof a threaded part of the aforementioned prestressing rod and forming alocking counter-nut. The prestressing rod can be chosen from the groupincluding at least screws, bolts, pins, and the compensating member canbe chosen from the group including at least springs, elastic washers,wedges of elastomer, and leaf springs.

In certain cases, the strain-measuring device can include a singledeformable element arranged between two mounting contacts approximatelyparallel to the fixation surface of these contacts and arranged fordeforming itself in a plane oriented perpendicular to this surface. Inthis case, the prestressing means can include two prestressing rods or aprestressing rod and a guiding rod, arranged parallely and symmetricallywith respect to the median plane passing through this deformableelement.

In other cases, the prestressing means can include a single prestressingrod arranged in the median plane passing through the aforementioneddeformable element. This deformable element can then include a centralhollow of traverse dimensions superior to those of the prestressing rod.

In still others cases, the measuring device can include two deformableelements arranged between two identical mounting contacts, symmetricallywith respect to a median plane of the aforementioned device,approximately perpendicular to the fixation surface of these contactsand arranged for deforming themselves in a plane approximately parallelto this surface. In this case, the prestressing means include a singleprestressing rod in this median plane.

To measure deformations in at least two directions, the strain-measuringdevice can include at least two and preferably three distinct mountingcontacts and a common mounting contact.

In one of the variant implementations, the strain-measuring deviceincludes at least a deformable element extending between theaforementioned common mounting contact and each distinct mountingcontact, the aforementioned deformable elements being angularly shiftedfrom an angle α, this angle having a value chosen from the groupincluding at least 30°, 45°, 60°, 90° and 120°.

In another variant implementation, the strain-measuring device includesat least a deformable element extending between each pair of distinctmounting contacts, the aforementioned deformable elements being arrangedapproximately in a triangle.

The deformable element is, preferably, constituted of an elastic stripmade of a material chosen from the group including at least stainlesssteel with or without structural hardening, titanium alloys, and copperalloys with beryllium. The deformable element and the mounting contactscan be advantageously formed from a single piece in a material of whichthe coefficient of dilation is close to that of the structure to bemeasured, this material being chosen from the group above including alsoaluminum alloys having a high elastic limit. In this case, theaforementioned deformable element is manufactured or cut from a materialto give it an initial curvature in a direction of its prestresscurvature.

According to the predicted results, the deformable element can bemounted by at least one of its ends in one of the mounting contacts byat least one technique chosen from the group including at least fitting,interlocking, screwing, riveting, gluing, and welding. Theaforementioned mounting contact can include a mounting zone forreceiving the aforementioned end of the deformable element, thismounting zone being advantageously inclined with respect to the fixationsurface of the aforementioned mounting contact in a manner to give theaforementioned deformable element an initial curvature in a direction ofits prestress curvature.

According to the applications, the mounting contacts are advantageouslyintended to be fixed on the aforementioned structure to be measured byat least one technique chosen from the group including at leastscrewing, riveting, gluing, and welding.

As a function of the technique used, the means for measuring stressundergone by the aforementioned deformable element are chosen from thegroup including at least resistive stress gauges, piezoelectric sensors,vibration sensors, and proximity contact sensors.

In a preferred mode, there are four stress gauges in a Wheatstonebridge.

The strain-measuring device preferably includes a protective housing atleast partially covering the mounting contacts and the deformableelements(s), this housing being able to be waterproofed as a function ofthe environment.

The means for treating signals includes at least an electronicconditioning circuit, this circuit being able to be integrated orcoupled to the aforementioned housing, or displaced and linked to theaforementioned housing by communication means. These means for treatingcan also be arranged for measuring the internal temperature of thedevice and correcting the values of the signals as a function of thistemperature.

The present invention and its advantages will be more apparent in thefollowing description of, non limiting, implementation examples inreference to the attached drawings, in which:

FIG. 1 is a longitudinally cut view of a first implementation form ofthe unidirectional strain-measuring device according to the invention,

FIGS. 2A, 2B and 2C are traversaly cut views of the device of FIG. 1according to the cut lines AA, BB and CC,

FIG. 3 is a view similar to FIG. 1 of a second implementation form ofthe device of the invention,

FIGS. 4 to 6 are plan cut views of three other implementation forms ofthe device of the invention,

FIG. 7 is a schematic profile view of the device of the invention,

FIG. 8 is a schematic view of the electrical wiring of the stress gaugesused in the device of the invention,

FIGS. 9A, 9B, and 9C represent three implantation examples of the stressgauges of FIG. 8, and

FIGS. 10 and 11 represent, in plan view, two examples of a device formeasuring bidirectional stress according to the invention.

The mechanical design of the unidirectional strain-measuring device10-14 can take several implementation forms according to the precisionand the extent of the desired measurement. In all the implementationforms, the strain-measuring device 10-14 includes one or two prestresseddeformable elements 20-24 extended across at least two mounting contacts30, 40, which can be integrally joined to the deformable element(s) (notshown) or be formed from interrelated pieces as illustrated. At leastone of the ends of each deformable element 20-24 is fixed in one of themounting contacts 30, 40. These mounting contacts define by theirinferior face a fixation surface permitting them to be fixed on thesurface of a structure 1 to be measured by fixation screws 50 and/or bygluing. The tensile or compression stresses undergone by the structure 1to be measured cause differential displacements for the mountingcontacts 30, 40. These displacements deform the deformable element(s)20-24 by augmenting or reducing its or their radius of curvature. Onerecords the deformation of each deformable element 20-24 with the aid ofmeasuring means 60 such as resistive stress gauges glued on one of thefaces or on the two faces of each deformable element, a displacementsensor storing its height, a piezo-electric sensor arranged, as statedbelow, in at least one of the mounting contacts for recording thevariation of the reaction force of each deformable element, etc. One canalso excite the deformable element(s) into vibration and measure its ortheir own frequency with the aid of a vibration sensor. The signalsfurnished by the measuring means 60 are analyzed by the treating means70, integrated, coupled, or displaced.

The unidirectional strain-measuring device of the invention isdistinguished from the prior art by the prestressing means 80, which areextended between the two mounting contacts 30, 40 and which are arrangedfor drawing together these two mounting contacts and imposing a preciseand regulatable curvature of prestress with the deformable element(s)20-24. These prestressing means 80 are also arranged for permitting arelative displacement between the two mounting contacts when thedeformable element(s) 20-24 are prestressed.

More specifically referring to FIGS. 5 and 6, the unidirectionalstrain-measuring device 13, 14 is also distinguished from the prior artin that it includes two deformable elements 23, 24 symmetric withrespect to a median plane and arranged approximately perpendicular tothe fixation surface of the contacts and thus to the structure 1 to bemeasured, so that they are deformed in a plane parallel to this surface.Conversely, in the other implementation forms referring to FIGS. 1 to 4,the strain-measuring device 10-12 includes a central deformable element20-22, arranged approximately parallel to the fixation surface of thecontacts, so that it is deformed in an plane perpendicular to thissurface.

Finally and referring to FIGS. 10 and 11, the strain-measuring devices15 and 16 are more particularly distinguished from the prior art in thatthey integrate in a single piece several unidirectional measuring devicepermitting measurement of deformations in at least two directions.

In the illustrated examples, the deformable element(s) 20-24 areconstituted by at least an elastic strip made of stainless steel with orwithout structural hardening, of an alloy of titanium, copper, orberyllium, etc., possibly forming a single piece with the mountingcontacts 30, 40. The prestressing means 80 are formed from at least aprestressing rod 81 provided at least with a head 82 at an end and athreaded part 83 at the other end coupled to a nut 84 cooperating with acompensation member 85. This prestressing rod 81 can be a bolt, a screw,a pin, a rod with two inverse threads. The nut 84 can be integrated,coupled to or made of a tapping in the mounting contacts 30, 40, and thecompensation member 85 can be a spring of compression or pullingaccording to its positioning, a stack of elastic washers, a wedge ofelastomer, one or several leaf springs and any equivalent elastic meansof which the stiffness can be chosen and/or adjusted according to need.The combination formed by the prestressing rod 81 and the nut 84 canalso be replaced by an axle coupled to an eccentric, a cam, cogs, or anyequivalent means allowing displacement in translation a mounting contactwith respect to the other in a precise and reproducible manner. If themeasuring means 60 are constituted of piezoelectric sensors, the lattercan be positioned under the head 82 of the prestressing rod 81, underthe compensation member 85 or under the nut 84.

A first implementation form of the strain-measuring device 10 isillustrated in FIGS. 1 and 2, FIG. 1 being a longitudinally cut view ofthis device according to a cutting plane passing through a prestressingrod 81 and FIGS. 2A to 2C being traversaly cut views according tocutting planes passing respectively though the fixation screw 50 of themounting contact 30 on the structure 1 to be measured, through thedeformable element 20 and through the fixation screws 51 of thedeformable element 20 in the mounting contact 40. In this example, thedeformable element 20 has an approximately rectangular form. Each end ofthis deformable element 20, formed by the short sides of the rectangle,is fitted in a slit 31, 41 provided in the corresponding mountingcontacts 30, 40 and fixed by two fixation screws 51. These slits 31, 41are inclined with respect to the fixation surface of the contacts forimposing on this deformable element 20 an initial curvature in thedirection of its prestress curvature. The fixation of the ends of thedeformable element 20 can also be obtained by any other means such aslaser welding, electron bombardment, direct clamping of the two fixationscrews, etc. These other fixation means allow, if necessary, toeliminate the slits 31, 41 costly to manufacture in the mountingcontacts 30, 40. In all these cases, the mounting contacts 30, 40include a mounting zone inclined with respect to their fixation surfacefor giving to the deformable element 20 an initial curvature in thedirection of its prestress curvature.

The prestressing means 80 of the deformable element 20 include twoprestressing rods 81 arranged in parallel and symmetrically with respectto the deformable element 20 to uniformly distribute the forces and toimpose on the deformable element 20 a deformation by buckling in asingle direction, this direction being predefineable by the initialcurvature, which is given to it at the time of its assembly in themounting contacts 30, 40. Each prestressing rod 81 freely traverses themounting contact 30 arranged near its head 82 through a straight boring32 and is screwed in a tapping 42 provided in the other mounting contact40, this tapping 42 constituting the nut 84. A counter-nut 86 isprovided at the end of the prestressing rod 81 for avoiding all risk ofunscrewing and thus of unsetting of the prestress. For reducing the costof manufacture, each prestressing rod 81 can freely traverse themounting contacts 30 and 40 through the straight borings 32 and 42 to bescrewed in the nut 86 forming the nut 84 coupled for example in a cavityblocking it from rotating provided in the mounting contact 40.

One of the two prestressing rods 81 can be replaced by a simple smoothguiding rod. The presence of the two prestressing rods 81 simplifies theimplementation by allowing utilization of identical pieces. Thisimplementation allows precise regulation of the prestress curvature ofthe deformable element 20 and makes this regulation reproducible sincethe distance between the two mounting contacts 30, 40 is measurable andcan be reproduced easily. Furthermore, the prestressing rods 81 allowmechanical tying of the two mounting contacts 30, 40 avoiding any riskof unsetting. Furthermore, this implementation allows balancing offorces by functioning on the fixation points of the mounting contacts30, 40 on the structure 1 to be measured, making the interactions withthis structure very weak, and (allowing) conservation of the regulationof the prestress during transportation or in case of dismounting of thestrain-measuring device 10. A compensating member 85 is coaxiallyarranged on each prestressing rod 81 between its head 82 and themounting contact 30 in a cavity 33 provided for this effect. It allows arelative movement between the two mounting contacts 30, 40 so that theycan follow deformations of the structure 1 to be measured.

The treatment means 70 of the signals emitted by measuring means 60 areconstituted by an electronic signal conditioning circuit placed directlyabove the deformable element 20 and mounting contacts 30, 40. It allowsintegration of the circuits for regulation of voltage, for regulation ofgain, for shifting of offset and for thermal compensation withoutproviding resistances for compensation of temperature on the deformableelement 20. However, it is possible to add such resistances fortemperature, according to arrangements well known to the person skilledin the art, for improving the precision of the thermal compensation.

This strain-measuring device 10 is lodged in a protective housing 90,which can be waterproofed or not according to the environment in whichit is placed. In the illustrated example the protective housing 90 isnot waterproofed and includes a cover 91 covering the upper and sideparts of the strain-measuring device 10 as well as a plaque 92 arrangedbetween the two mounting contacts 30, 40. The protective housing 90remains open at its ends to allow a lap of conductive wires 71 allowingelectrical supply of the stress gauges, connection of the electronicconditioning circuit 70 to a signals reading apparatus and/or to aninformation management unit for example (not shown). The cover 91 can bejoined to the mounting contacts 30, 40 by interlocking, clamping orcasting. The plate 92 can serve as a buttress to avoid deterioration ofthe deformable element 20 by excessive tightening of the prestressingmeans 80.

Mounting and adjustment of the strain-measuring device 10 is realized inthe following manner. The deformable element 20 is mounted in themounting contacts 30, 40 by fitting its ends in the slits 31, 41 thensolidly fixed by the four screws 51. The inclination of the slits 31, 41elastically deforms the deformable element 20, which takes an initialcurvature. The prestressing rods 81 with their compensating member 85are engaged in the mounting contact 30 then screwed in the mountingcontact 40. The rotation of the compensation rods 81, by means of a tooladapted to the impression provided in the head 82, has the effect ofcompressing the compensation members 85 and pulling mounting contacts30, 40 toward each other, driving the bending and the augmentation ofthe radius of curvature of the deformable element 20 thus causing itsprestress. The presence of the compensation members 85 makes theadjustment finer. Their reaction allows balance of the horizontal forcesexerted by the deformable element 20 on the mounting contacts 30, 40.The guidage of the mounting contacts 30, 40 by the prestressing rods 81balances the moments imposed on these contacts and caused by thebuckling of the deformable element 20. This guidage imposes on thedeformable element 20 a deformation in a single direction generating adrawing together of their two points of fixation, thus of the twomounting contacts 30, 40, of a certain value. Knowledge of this valuepermits the reproducibility of the prestress adjustment of thedeformable element 20.

Once the strain-measuring device 10 is preadjusted, one can glue itand/or screw it onto the surface of a structure to be measured for thepurpose of measuring the deformations of this structure, or beforehandon a standard piece for the purpose of calibrating and adjusting theelectrical characteristics of the measuring means 60. The compensationmembers 85, opposing the horizontal forces of the deformable element 20,reduce the forces in their fixation points in the mounting contacts 30,40. Nevertheless, the elasticity of these compensation members 85 allowsthe functioning of the strain-measuring device 10 without needing towithdraw the prestressing rods 81. The particular disposition of theseprestressing rods 81 allows utilization of fixation screws 50 of reducedcross section or simple gluing without risk of creeping of thestrain-measuring device 10.

This first implementation form, which uses a hyperstatic assembly of thedeformable element 20, allows achievement of a strong sensitivity, agood measurement precision and good reproducibility of the prestress. Onthe other hand, it limits the extent of measurement, considerablycomplicates the assembly and can present large thermal drifts. In fact,the hyperstatic assemblies permit no degree of freedom for the dilationsand thus lead to elevated thermal stresses.

A second implementation form of a strain-measuring device 11 isrepresented in FIG. 3, which is a longitudinally cut view according to acutting plane passing through fixation screws 50 of the device on thestructure 1 to be measured. In this variation, the deformable element 21is fitted to a single side in a slit 41 of the mounting contact 40,forming a “cantilever” beam. The other end is retained in the othermounting contact 30 by a lip 34 of a superior support. As in thepreceding example, the slit 41 is inclined with respect to the fixationsurface of the contacts to give the deformable element 21 an initialcurvature in a direction of its prestress curvature. This assembly isisostatic and allows a larger extent of measurement with equaldimensions and a reduced sensitivity to the thermal variations becauseit offers a larger degree of freedom. The protective housing 90 includestwo end walls 93 that approximately seal openings left between themounting contacts 30, 40 and the cover 91. One of the end walls 93allows the passage of a lap of conductive wires 71 from the electronicconditioning circuit 70.

Three other implementation forms are represented respectively in FIGS. 4to 6, in which the strain-measuring device 12,13, 14 includes only asingle prestressing rod 81, arranged in the median plane of the device.These FIGS. 4 to 6 are plan cut views according to a cutting planepassing through this prestressing rod 81 and showing threeimplementation examples of the deformable element 22, 23, 24.

In the third implementation form referring to FIG. 4, thestrain-measuring device 12 includes a deformable element 22 having anapproximately rectangular shape larger than precedents 20, 21. Thisdeformable element 22 includes a central hollow 22′ having a widthgreater than the diameter of the prestressing rod 81 allowing it to passwhen the deformable element 22 is prestressed. The ends of thisdeformable element 22 form a larger seat in the mounting contacts 30, 40and secure a deformation of the deformable element 22 in a singledirection, this direction being predefineable by an initial curvaturegiven to this deformable element 22 at the time of its assembly. Thisconstruction being balanced in the horizontal plane, a singleprestressing rod 81 is necessary. The central hollow 22′ lessens thecross section of the deformable element 22 and avoids exaggeratedaugmentation of the horizontal forces in its fixation points. The shapeof this deformable element 22 can present advantages for implantation ofstress gauges 60 (not shown in this Figure). Its fixation in themounting contacts 30, 40 can be implemented according to the hyperstaticassembly of FIG. 1 or according to the isostatic assembly of FIG. 3.

In the fourth implementation form referring to FIG. 5, thestrain-measuring device 13 includes two identical deformable elements 23mounted symmetrically with respect to the median plane of thestrain-measuring device 13, on both sides of the prestressing rod 81. Asin the preceding example, the objective of this realization is tobalance the reaction forces of the deformable elements in the mountingcontacts 30, 40. In this implementation, the deformable elements 23 aremounted on their side and their ends are fitted in slits 31, 41 providedin the mounting contacts 30, 40 and inclined with respect to the medianplane to give to the deformable elements 23 an initial curvature in adirection of their prestress curvature. In this assembly, the reactionforces of the two deformable elements 23, from the vertical axis, tendto turn the mounting contacts 30, 40 in the horizontal plane, mutuallybalancing themselves. The compensating member 85 compensates thehorizontal reactions in the same manner and the sum of these reactionsexerted on the structure to be measured is null when thestrain-measuring device 13 is in initial position. The axial guidage ofthe prestressing rod 81 in the mounting contacts 30. 40 becomesunnecessary because of the perfect symmetry of assembly permittingbalancing of the forces. The only force exerted on the structure to bemeasured is the difference between the reaction of the deformableelements 23 and of the compensating member 85 for a given functioningposition.

In the fifth implementation form referring to FIG. 6, thestrain-measuring device 14 differs from the preceding one in that thetwo mounting contacts 30, 40 and the two deformable elements 24 areformed by a single piece plate of small width. This variation isadvantageous from an economic point of view considering the reduction ofthe number of pieces and the ease of assembly as well as the mechanicalpoint of view considering the suppression of inevitable non-linearitiesin any assemblage. Furthermore, it can be realized at lower cost, thispiece being able to be obtained by cutting from laminated sheets oradjusted by any known means such as wire electroerosion, water jet, toolcutting, etc. To realize this piece, one will preferably choose amaterial of which the coefficient of dilation is close to that to thestructure to be measured 1, such as for example a stainless steel, asteel with a high elastic limit of type Z200 C12, a steel withstructural hardening, the titanium alloys, the cooper alloys withberyllium, the aluminum alloys with a high elastic limit, etc. In thisform of realization, the deformable elements 24 are manufactured or cutaccording to a curve to give them an initial curvature in a direction oftheir prestress curvature.

Whatever the form of realization of the strain-measuring device 10-14according to the invention, the deformation of the deformable element(s)20-24 is similar.

Each deformable element 20-24 presents, referring to the schematic ofFIG. 7, a convex face A and a concave face B. The convex face Aintroduces positive tensile stresses in the median zone and negativecompression stresses in the fitting zones. On the concave face B, thesigns of the stresses are inverted. Several implantations of themeasuring means 60 are thus possible. One introduces only theaforementioned complete bridge mountings or Wheatstone bridge using fourstress gauges 61, 62 allowing acquisition of the strongest and leastperturbable signal. The four stress gauges 61, 62 are connectedaccording to the schematic of FIG. 8. The signals of the two stressgauges 61 or 62 are additionally opposed and the signals of two adjacentgauges 61, 62 are subtracted, which allows by application on thepositions of the stress gauges 61, 62 acquisition of interestingcombinations of signals. In the implementations of FIGS. 1 to 4, thefour gauges 61, 62 are distributed on one or the two faces of thedeformable element 20-22 and, in the implementations of FIGS. 5 and 6,the four gauges 61, 62 are distributed two by two on one or the twofaces of each of the deformable elements 23, 24.

FIGS. 9A to 9C illustrate three possible arrangements of the stressgauges 61, 62. In FIG. 9A, the four stress gauges 61, 62 are arranged ona single face A or B of the deformable element. They are positionedsquarely side by side and are inverted one with respect to one another.It concerns a “traction bridge” arrangement. The two longitudinallyoriented stress gauges 61 absorb the principal deformation and twotraversely oriented stress gauges 62 record a inverse sign contraction,called the “Poisson effect”. The signal with respect to a single stressgauge 61, 62 is multiplied by 2.6. This arrangement requires very smallstress gauges 61, 62 and is particularly advantageous in the case ofstress gauges arranged in thin layers.

In FIG. 9B, one glues the stress gauges 61, 62 longitudinally alignedtwo by two in the middle of each face A and B of the deformable element20-24.

This arrangement has the advantage of providing a stronger signal andallowing utilization of larger stress gauges 61, 62. However theirpositioning is complex.

In FIG. 9C, one arranges the four stress gauges 61, 62 on a single faceA or B of the deformable element 20-24. They are rectangularlypositioned and oriented longitudinally. This arrangement proves to bethe most interesting because it permits absorption of a signal almost asstrong as with the arrangement of FIG. 9B, all in not requiring except asingle face of the deformable element 20-24 by taking advantage of thechange of sign of the stresses near ends of the deformable element20-24.

In the strain-measuring devices 10-14 represented in FIGS. 1 to 7, thearrangement of the stress gauges 61, 62 in accordance with FIGS. 8 and 9is such that the deformable element(s) 20-24 are not sensitive except tothe linear deformations of the structure 1 to be measured, that is tosay oriented in the longitudinal axis of each deformable element 20-24.This implies knowing in advance the direction of the deformation to bemeasured.

In the strain measuring devices 13 and 14 of FIGS. 5 and 6, one caneasily arrange the stress gauges 61, 62 in such a manner that thedeformable elements 23 and 24 are sensitive to distortion (shearingforces). For this effect, one can connect, referring to FIG. 9C, the twostress gauges 61 of the middle in two adjacent branches of the Wheatstonbridge and the stress gauges 62 of the end in the two remaining branchesof the bridge.

In the hypothesis where the direction of the deformations to be measuredis unknown, the strain-measuring device must be able to detectbi-directional deformations contained in a plane by means of two orthree deformable elements oriented according to different directions. Itis necessary in effect to be able to determine three unknowns: the twoprincipal elongations and their angle of orientation in a common plane.

The original design of the strain-measuring device 10-14 according tothe invention enables providing a simple and economical solutioncombining two or three similar devices in one. FIG. 10 illustrates animplementation example of a bi-directional strain-measuring device 15combining three strain-measuring devices 14 of FIG. 6. The threestrain-measuring devices 14 are assembled in one around a commonmounting contact 40 and arranged at the center. The three other mountingcontacts 30 are arranged at equal distance from the central contact 40and are angularly shifted from each other at an angle α approximatelyequal to 120° in a manner to cover 360°. Other configurations arepossible. Preferably, one chooses values of the angle convenient forcalculations, as for example 45°, in which case the device covers 90°,or 60° in which case the device covers 120°.

FIG. 11 illustrates another implementation example of a bi-directionalstrain-measuring device 16 also including three mounting contacts 30arranges at equal distance from a central contact 40 and angularlyshifted from each other at an angle α approximately equal to 120° in amanner to cover 360°. The difference resides in the deformable elements24, which are three in number and are each extended between two mountingcontacts 30 for approximately forming a triangle or a delta.

Implementing the bidirectional strain-measuring device 15-16 using thetechnology used in the device 14 enables manufacturing simply and atlower cost the combination formed by the mounting contacts 30, 40 andthe deformable elements 24 in a common cut piece. The deformableelements 24 are, preferably, manufactured or cut according to a slightcurve to give them an initial curvature in a direction of theirprestress curvature. They are prestressed independently or by pair bymeans of one or two prestressing rods 81, each screwed in a connectednut 84 and lodged in a hollow 43 provided in the central assemblycontact 40. This prestress is of course reproducible and reliable as inall the variants of realization described above. In the strain-measuringdevice 16, the common, central mounting contact 40 can be not glued orfixed on the structure 1 to be measured.

Each deformable element 24 is equipped with stress gauges 61, 62 andfunction as a unidirectional sensor. One then utilizes the propertiesknow by the person skilled in the art of Mohn's circle to determine themodules and the directions of the deformations of the structure 1 to bemeasured. One can reconstruct the matrix of the deformation in a pointin the direction of the deformable elements 24. In this particularimplementation, one generates three different signals that one can treatsubsequently or directly by the electronic conditioning circuit thatthen integrates the calculations necessary to the treatment of thesignals.

Each deformable element 24 can receive four stress gauges 61, 62positioned and cabled to be sensitive to the longitudinal deformationsand at least one of the deformable elements 24 can receive four stressgauges 61, 62 positioned and cabled to be sensitive to the shear. Inthis case and only for the strain-measuring device 15, it is possible toutilize only two strain measuring devices 14 arranged at a right angle.

It follows clearly from the present description that the inventionenables achievement of the fixed goals. The particular design of thestrain-measuring device 10-16 enables very simple implementations andutilizations thanks to optimization of certain characteristics:

-   -   Setting of the strain-measuring device 10-16 on the structure 1        to be measured is very rapid, economical, and does not require        intervention of a specialized technician.    -   The setting does not require any modification of the structure 1        to be measured.    -   The stiffness of the measuring device 10-16 is very low in the        face of the structure 1 to be measured thus has no influence on        it.    -   The balancing of the forces of the fixation points enables        setting of the measuring device 10-16 on the structure 1 to be        measured solely with the aid of small fixation screws 50 or        simple gluing.    -   The strain-measuring device 10-16 can be made entirely        waterproof in case of severe environmental conditions.    -   The electronic conditioning circuit 70 can be integrated.    -   Numerous characteristics can be programmed before the use of the        measuring device 10-16, such as the relation between the        measured deformation and the output electric signal, and the        compensation of the dilation of the deformable element 20-24.    -   Certain characteristics can be remotely reprogrammed without        mechanical interventon on the measuring device 10-16, such as        the value of the shift, a precise recalibration in situ, etc.

Of course, the present invention is not limited to the describedimplementation examples but extends to all modification and variationapparent to a person of skill in the art all remaining within the scopeof the protection defined in the annexed claims.

1. A strain-measuring device (10-16) including at least a deformableelement (20-24) placed between at least two mounting contacts (30, 40)defining a fixation surface for being fixed on a structure (1) to bemeasured, prestressing means (80) of the aforementioned deformableelement (20-24), measuring means (60) of the stress undergone by theaforementioned deformable element (20-24) and treatment means (70) ofmeasurement signals, characterized in that the prestressing means (80)are extended between at least the two mounting contacts (30, 40) and arearranged for drawing together these two mounting contacts (30, 40) intranslation and imposing a curvature of precise prestress to theaforementioned deformable element (20-24), these prestressing means (80)being also arranged for allowing a relative displacement in translationof these two mounting contacts (30, 40) when the aforementioneddeformable element (20-24) is prestressed.
 2. The device according toclaim 1, characterized in that the prestressing means (80) are freelymounted at least in translation in one of the mounting contacts (30) andare tied at least in translation to the other mounting contact (40). 3.The device according to claim 2, characterized in that the prestressingmeans (80) includes at least a prestressing rod (81), of which at leastthe part (83) tied in translation to the aforementioned mounting contact(40) is threaded and cooperates with at least a nut (84) for displacingthe aforementioned mounting contact (40) and at least a compensationmember (85) arranged for exerting a return force between theaforementioned prestressing rod (81) and one of the mounting contacts(30).
 4. The device according to claim 1, characterized in that theprestressing means (80) are tied at least in translation to the twomounting contacts (30, 40).
 5. The device according to claim 4,characterized in that the prestressing means (80) includes at least aprestressing rod, of which at least the parts tied in translation to theaforementioned mounting contacts (30, 40) are threaded in an oppositedirection, at least two nuts arranged for receiving the aforementionedthreaded parts from the prestressing rod and at least a compensationmember arranged for exerting a return force between the aforementionedmounting contacts (30, 40). 6-24. (canceled)
 25. The device according toclaim 1, characterized in that the aforementioned deformable element(20-23) is mounted by at least one of its ends in one of the mountingcontacts (30, 40) by at least a technique chosen from the groupcomprising at least fitting, interlocking, screwing, riveting, gluing,and welding.
 26. The device according to claim 25, characterized in thatthe aforementioned mounting contact (30, 40) comprises a mounting zone(31, 41) for receiving an end of the deformable element (20-23), thismounting zone being inclined with respect to the fixation surface of theaforementioned mounting contact (30, 40) in a manner to give to theaforementioned deformable element (20-23) an initial curvature in adirection of its prestress curvature.
 27. The device according to claim1, characterized in that the aforementioned mounting contacts (30, 40)are for fixing on the aforementioned structure (1) to be measured by atleast a technique chosen from the group comprising at least screwing,riveting, gluing, and welding.
 28. The device according to claim 1,characterized in that the measuring means (60) of the stress undergoneby the aforementioned deformable element (20-24) are chosen from thegroup comprising at least resistive stress gauges (61,62),piezo-electric sensors, contactless proximity sensors, and vibrationsensors.
 29. The device according to claim 28, characterized in that theaforementioned stress gauges (61,62) are four in number and mounted as aWheatstone bridge.
 30. The device according to claim 1, characterized inthat it comprises a protective housing (90) at least partially coveringthe aforementioned mounting contacts (30, 40) and the aforementioneddeformable element(s) (20-24).
 31. The device according to claim 27,characterized in that the aforementioned protective housing iswaterproofed.
 32. The device according to claim 27, characterized inthat the treatment means (70) of the signals comprises at least anelectronic conditioning circuit, this circuit being integrated with orcoupled to the aforementioned housing (90), or displaced and linked tothe aforementioned housing by means of communication.
 33. The deviceaccording to claim 1, characterized in that the aforementioned treatmentmeans (70) of signals are arranged for measuring the internaltemperature of the aforementioned device and correcting the values ofthe aforementioned signals as a function of this temperature.